Focused ion beam (FIB) systems are widely used in characterization or treatment of materials on microscopic to nanoscopic scales. For example, focused ion beam systems are used in manufacturing operations because of their ability to image, mill, deposit and analyze with great precision. Ion columns in FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometers of lateral resolution. Such focused ion beam systems are commercially available, for example, from FEI Company, Hillsboro, Oreg., the assignee of the present application. Although use of liquid metal ion sources has increased, their application is often limited due to metal ion contamination and relatively low obtainable beam currents. Lower beam currents result in lower erosion rates and hence longer processing times in production applications and in laboratories.
In contrast to FIB systems are broad ion beam systems suitable for semiconductor processing on a relatively large scale. For example, a broad beam system may be used for semiconductor doping over nearly the entire surface of a silicon substrate wafer. RF sources have been used for large-area wafer processing. For such uses no probe forming optics are employed.
A challenge exists in generating high current, low energy beams for implantation from an RF plasma source. A complication associated with using an RF driven ion source for low energy ion implantation is the undesirable oscillations imparted to the plasma potential, through capacitive coupling from the antenna to the plasma. The plasma potential can have peak-to-peak oscillations of several hundred volts, thereby dramatically modulating the extracted beam energy. Such a highly modulated beam is unsuitable for low energy ion implantation, due to the broadened projected ion range into the target.
Ion energy modulation is even less suitable for FIB systems, due to the associated chromatic aberrations generated in the beam. The relationship between the energy spread due to RF modulation of the beam and the chromatic blur of the beam is given by:
                              d          c                =                                            Δ              ⁢                                                          ⁢              E                                      E              0                                ⁢                      C            c                    ⁢                      α            i                                              (        1        )            where, dc is the diameter of the chromatic disc, Cc is the chromatic aberration coefficient for the optical system and αi is the convergence half-angle of the beam as it forms the focused spot at the target. Eo is the energy to which the ions are accelerated by the extraction optics. The term ΔE is the energy spread that results from modulations in the plasma potential due to capacitive coupling from the antenna, coupled with the fundamental axial energy spread of ions from the source that is determined by the potential gradient in the pre-sheath region of the plasma. Clearly, the modulations imparted by the RF source substantially and undesirably impact the focus of the beam. At least partially for this reason, it is believed that RF sources have not been successfully used with FIB systems.
For precision milling and deposition, one desires high beam current for faster production times, focused into a small spot. Hence, high source brightness and minimal optical aberrations are required. The “brightness” of a beam from a plasma source is proportional to the beam current density from the source and inversely proportional to the mean thermal ion energy for ions existing in the plasma. This is expressed by the equation (2):
                              β          max                =                                            J              i                        ⁢                          E              0                                            π            ⁢                                                  ⁢                          E              ⊥                                                          (        2        )            Here βmax is the beam brightness assuming zero aberrations introduced from the extraction optics, Ji is the current density extracted from the plasma, Eo is the energy to which the ions are accelerated by the extraction optics and E⊥ is the mean thermal ion energy. Clearly, beam brightness increases when the current density is increased and the mean thermal ion energy is decreased.
The current density, Ji, is given by:
      J    i    =      0.6    ⁢                  ⁢          n      i        ⁢    q    ⁢                                        k            B                    ⁢                      T            e                                    M          i                    where, (ni) is the plasma ion density, (Te) is the mean electron energy within the plasma, (q) is the fundamental unit of charge, (kB) is Boltzmann's constant, and (Mi) is the mass of the of the ion in the plasma. Clearly, current density is increased by increasing the plasma ion density and increasing the mean electron energy in the plasma. Hence, for the values indicated above, the optimum αi is determined to be ˜7.5 mrads, resulting in an image side brightness of ˜7×103 A cm−2 sr−1 and a source brightness of ˜1.5×104 A cm−2 sr−1 at 20 keV.
To summarize, we want high current density and low thermal energy to obtain high beam brightness. To achieve high current density, we want high plasma ion density and high mean electron energy. What is needed, therefore, is a high-density focused plasma ion beam system with low thermal ion energies to facilitate high brightness.